Haematology
Iron Deficit (Ganzoni)
Total iron deficit for replacement.
Education and reference only. Not a substitute for clinical judgement, local policy or product labelling. Always verify before clinical use. Values are calculated in your browser and never stored.
Background
The Ganzoni formula estimates the total amount of iron required to correct iron-deficiency anaemia and to replenish body iron stores, typically when planning intravenous iron replacement. It accounts for the patient's weight, the gap between their actual and target haemoglobin, and a fixed allowance for iron stores. The formula is: iron deficit (mg) = weight (kg) × (target − actual haemoglobin, g/dL) × 2.4 + iron stores. The result is the total iron requirement in milligrams.
Interpreting the result
The output is a target total iron requirement, not a single injection amount, and it rises with greater weight and a larger haemoglobin deficit. For adults of 35 kg or more, a store allowance of approximately 500 mg is conventionally added, while for those under 35 kg a smaller weight-based allowance (about 15 mg/kg) is used. The calculated deficit guides the overall replacement plan; how it is divided and delivered depends on the specific intravenous iron product chosen.
Worked example
A 70 kg adult with an actual haemoglobin of 8 g/dL and a target of 12 g/dL, with a 500 mg store allowance: 70 × (12 − 8) × 2.4 + 500 = 672 + 500 = a total iron deficit of about 1,172 mg.
Critical actions
Ganzoni: deficit = weight × (target − actual Hb) × 2.4 + stores. Hb in g/dL; stores ≈500 mg for adults ≥35 kg (use 15 mg/kg if <35 kg). Check the specific IV-iron product's own dosing table too.
Pearls / pitfalls
- The result is a total target requirement; consult the specific intravenous iron product's own dosing and maximum single-dose information when delivering it.
- Use a store allowance of about 500 mg for adults of 35 kg or more, and roughly 15 mg/kg for those under 35 kg.
- The formula can underestimate requirements in some patients, and newer fixed-dose regimens are increasingly used instead.
- Confirm true iron deficiency (for example with ferritin and transferrin saturation) before calculating and replacing.
Evidence & validation
The formula was described by Ganzoni and has been used for decades to plan parenteral iron replacement. It is referenced in haematology and gastroenterology guidance on managing iron deficiency, although modern intravenous iron products also provide their own simplified dosing tables.
Frequently asked questions
What does the Ganzoni formula calculate?
It calculates the total iron deficit in milligrams needed to correct anaemia and replenish stores, based on weight, the haemoglobin gap and a store allowance. It gives an overall target requirement rather than a single injection amount.
How much is added for iron stores?
Conventionally about 500 mg is added for adults weighing 35 kg or more, and roughly 15 mg per kg for those under 35 kg. This allowance ensures that stores, not just circulating haemoglobin, are replenished.
Is the Ganzoni formula still used?
It remains a recognised way to estimate the iron deficit, but many modern intravenous iron preparations come with their own simplified, weight- and haemoglobin-based dosing tables, which are often used in preference.
Should I confirm iron deficiency before using it?
Yes. The formula assumes iron-deficiency anaemia, so confirm the diagnosis with appropriate iron studies first. Replacing iron in anaemia from another cause would be inappropriate and potentially harmful.
References
- Ganzoni AM. Intravenous iron-dextran: therapeutic and experimental possibilities. Schweiz Med Wochenschr. 1970;100(7):301–303.
- Goddard AF, James MW, McIntyre AS, Scott BB; British Society of Gastroenterology. Guidelines for the management of iron deficiency anaemia. Gut. 2011;60:1309–1316.
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